1. Field of the Invention
The present invention relates generally to semiconductor laser devices and methods of manufacturing the same and particularly it relates to an improvement of a compound semiconductor laser device which contains Al, Ga, In and P as major constituents for emitting the visible light, and a method of manufacturing the same.
2. Description of the Background Art
A metal organic chemical vapor deposition (MOCVD) method is an effective method for growing crystals of a GaInP system (or an AlGaInP system). However, many crystal defects are often observed in the crystals of the GaInP system grown by the MOCVD method. For example, when a crystal layer of the GaInP system is grown on the {100} plane of a GaAs substrate by the MOCVD method, about 6000 crystal defects of hillocks oval in section are generated per 1 cm.sup.2 on the crystal grown plane.
According to Journal of Crystal Growth, 17(1972), pp. 189-206, it is stated that the number of undesirable pyramid hillocks on the crystal grown surface of the GaAs layer grown by the CVD method on the GaAs substrate can be considerably decreased by using a substrate having a surface inclined by 2.degree. to 5` from the {100} plane in the &lt;110&gt; direction.
In addition, according to Journal of Crystal Growth, 68(1984), pp. 483-489, a semiconductor laser device of the AlGaInP system manufactured by using the MOCVD method is described.
FIG. 1 is a schematic sectional view of such a semiconductor laser device of the AlGaInP system. In this semiconductor laser device, one main surface 1a of an n type GaAs substrate 1 is inclined by 2.degree. from a {100} plane in a &lt;110&gt; direction. An n type GaAs buffer layer 2 of 0.7 .mu.m in thickness, an n type (Al.sub.0.3 Ga.sub.0.7).sub.0.5 In.sub.0.5 P clad layer 3 of 4 .mu.m in thickness, a non-doped Ga.sub.0.5 In.sub.0.5 P active layer 4 of 0.23 .mu.m in thickness, a p type (Al.sub.0.3 Ga.sub.0.7).sub.0.5 In.sub.0.5 P clad layer 5 of 1.4 .mu.m in thickness and a p type GaAs cap layer 6 of 1.0 .mu.m in thickness are stacked successively on the main surface 1a.
A current blocking layer 7 having a stripe-shaped opening 8 of a width of 20 to 23 .mu.m is formed on the cap layer 6. The cap layer 6 exposed in the blocking layer 7 and the opening 8 is covered with a p side Au/Zn electrode layer 9 including a Zn sub layer and an Au sub layer successively stacked. An n side Au/Ge/Ni electrode layer 10 including a Ni sub layer, a Ge sub layer and an Au sub layer successively stacked is formed on the other main surface 1b of the n type GaAs substrate 1.
Semiconductor laser devices as shown in FIG. 1 have disadvantages such as a large variation of oscillation threshold currents between the devices and a deteriorated yield. According to the investigation conducted by the inventors of the present invention, many hillocks were observed on the surface of the cap layer 6 of such a device.
More specifically, as described in Journal of Crystal Growth, 17(1972), pp. 189-204, the utilization of a surface inclined by 2.degree. to 5.degree. from a {100} plane of a GaAs substrate in a &lt;110&gt; direction is effective in suppressing hillocks in the growing process of a GaAs crystal layer by the CVD method but it is not effective in suppressing hillocks in the growing process of an Al GaInP system crystal layer by the MOCVD method.
FIG. 2 is a schematic sectional view showing another conventional semiconductor laser device. An n type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P clad layer 12, a non-doped (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P active layer 13, and a p type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P clad layer 14 are epitaxially grown successively on an n type GaAs substrate 11 by using the MOCVD method or molecular beam epitaxy (MBE) method or the like. Ridges of a width of about 5 .mu.m are formed by etching on the p type clad layer 14.
An n type GaAs current blocking layer 15 epitaxially grown by using a mask is formed on the p type clad layer 14. However, the top surfaces of the ridges of the p type clad layer 14 are not covered with the current blocking layer 14. The-top surfaces of the ridges of the p type clad layer 14 and the blocking layer 15 are covered with a p type GaAs cap layer 16 epitaxially grown.
A p side electrode layer 17 of Au/Zn/Au is formed on the cap layer 16. On the other hand, an n side electrode layer 18 of AuGe/Au is formed on the other main surface of the n type GaAs substrate 11.
When the Al composition ratio of the active layer 13 of the semiconductor laser device of FIG. 2 is x=0.1, a laser beam of a wavelength of 649 nm is obtained. On the other hand, a He--Ne gas laser device having a wavelength of 632.8 nm is used these days as a light source of a bar code scanner used in a measuring instrument or a point-of-sales (POS) system using visible laser beam. However, such a gas laser device has a large size and a heavy weight and it consumes much power. Accordingly, it is desired to use an AlGaInP system semiconductor laser device having a light weight and a small size with low consumption of power in place of a He--Ne gas laser device, by making a little shorter the wavelength of an AlGaInP system semiconductor laser device.
An AlGaInP system semiconductor laser device capable of emitting laser beam having a shorter wavelength can be obtained by taking one of the following measures:
(1) the composition ratio of Al in the active layer is increased; PA1 (2) the active layer is formed to have a superlattice structure (see Electronics Letters, Vol. 24, 1988, pp. 1489-1490); PA1 (3) each of the semiconductor layers is grown at a temperature higher than 700.degree. C. (see Japanese Journal of Applied Physics, Vol. 27, 1988, pp. 2098-2106); or PA1 (4) Zn is diffused in the active layer (see IEEE Journal of Quantum Electronics, QE-23, 1987, pp. 704-711).
For example, if measure (1) is used in the case of the composition ratio of Al in the active layer 13 being x=0.2, the oscillation wavelength is 630 nm to 640 nm, which value is substantially equal to the wavelength of a He--Ne laser beam. However, if the composition ratio of Al in the active layer 13 is increased, the quality of crystals of the active layer 13 is lowered and the oscillation threshold current is increased, making it difficult to carry out continuous operation of the semiconductor laser device. If either of the measure (2), (3) or (4) is taken, oscillation operation of the semiconductor laser device becomes unstable, resulting in a low yield of manufacturing and a considerable deterioration of the active layer 13, making the life of the device short.